(a) Field of the Invention
The present invention relates to ventilation valves, and particularly to a ventilation valve of a hydraulic jack, wherein when the jack is not operated, a valve hole is opened; the vent of a cap of the jack is communicated with the oil storage tank of the jack so as to maintain the equilibrium of the pressure of the oil storage tank; thereby the piston of the pump can suck oil successfully so that the oil is absorbed into the oil chamber of the pump to be saturated therein.
(b) Description of the Prior Art
In the lifting process of a prior art jack, a piston of a pump absorbs oil from an oil storage tank into an oil chamber of a pump. Then oil is transferred into an inner oil tank. By the reciprocal operation of above process, the jack can lift upwards gradually. In the prior art design, the oil storage tank of a hydraulic jack is sealed completely. When the pump operates initially, the oil in the oil storage tank passes through the oil chamber of the pump and enters into the inner oil tank. The oil storage will reduce gradually. The less the oil storage, the smaller the pressure of the oil storage tank. As a result, the pressure will approach to a vacuum pressure. Thereby, the pumping operation becomes difficult so that the piston can absorb oil successfully. Moreover, the oil absorption cannot be in an optimum saturation condition. Thus, in each lifting process, a maximum efficiency can not be achieved.